1. Field of the Invention
The present invention relates to a substrate having a silicon-on-insulator (SOI) structure where a semiconductor layer is formed by slicing of a crystalline semiconductor substrate and bonded to a substrate of a different kind. In particular, the present invention relates to bonding SOI technology and relates to a method for manufacturing an SOI substrate where a single-crystal or polycrystalline semiconductor layer is bonded to a substrate having an insulating surface such as a glass substrate. The present invention also relates to a display device or a semiconductor device using such a substrate having an SOI structure.
2. Description of the Related Art
Instead of silicon wafers that are manufactured by thinly cutting an ingot of a single-crystal semiconductor, semiconductor substrates called silicon-on-insulator (SOI) substrates have been developed, which have a thin single-crystal semiconductor layer over an insulating layer and are becoming widely used as substrates in manufacturing microprocessors or the like. This is because an integrated circuit using an SOI substrate has attracted attention since parasitic capacitance between drains of transistors and the substrate can be reduced, the performance of a semiconductor integrated circuit can be improved, and the amount of power consumption can be reduced.
As a method for manufacturing SOI substrates, a hydrogen ion implantation separation method is known. In the hydrogen ion implantation separation method, hydrogen ions are implanted into a silicon wafer to form a microbubble layer at a predetermined depth from the surface, and the microbubble layer is used as a cleavage plane to bond a thin silicon layer (SOI layer) to another silicon wafer. In addition to heat treatment for separation of an SOI layer, it is necessary to perform heat treatment in an oxidizing atmosphere to form an oxide film on the SOI layer, remove the oxide film, and perform heat treatment at 1000° C. to 1300° C. in a reducing atmosphere to increase bonding strength.
On the other hand, there has been an attempt to form an SOI layer over an insulating substrate such as a glass substrate. One known example of SOI substrates where an SOI layer is formed over a glass substrate is that in which a thin single-crystal silicon layer is formed over a glass substrate, which has a coating film, by a hydrogen ion implantation separation method (see Reference 1: Japanese Published Patent Application No. H11-163363). Also in this case, hydrogen ions are implanted into a piece of single-crystal silicon to form a microbubble layer at a predetermined depth from the surface; a glass substrate and the piece of single-crystal silicon are bonded together; and the microbubble layer is used as a cleavage plane to separate the piece of silicon, thereby forming a thin silicon layer (SOI layer) over a glass substrate.
In addition, techniques are under development in which a switching element or a driver circuit in an active-matrix electro-optical device, for example, an active-matrix electroluminescent (EL) display device is formed using a thin film transistor (TFT).